Electrical method of adhesive bond testing



March 13, 1951 a. D. HABER 2,544,673

ELECTRICAL METHOD OF ADHESIVE BOND TESTING Filed Jan. 16, 1948 Patented Mar. 13, 1951 UNITED STATES PATENT OFFICE ELECTRICAL METHOD OF ADHESIVE BOND TESTING Bernard D. Haber, Montgomery County, Ohio Application January 16, 1948, Serial No. 2,793 (ci. irs-9183 (Granted under the act of March 3, 1883, as

l Claims.

amended April 30, 1928; 370 O. G..'757) The inventio-n described herein may be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a method of. nondestructive testing of adhesive bonds. More particularly `it relates to an electrical method for testing whether thenbondV of cement which at- Fig. 6 is a cross section corresponding to Fig. 5 except that the condenser plate is on the bottom of theA structural member.

taches wire strainlgauges to the structures in which strains are to be measured, is satisfactory. Another application of the method is in testing the bond between the sheets of plywood.

One object of the invention is, therefore, to provide a convenient and non-destructive method of testing adhesive bonds. Other objects will be hereinafter apparent to those skilled in the art.

My method necessitates the use of an instrument which indicates Vminute changes in capacitance, such as a capacitance bridge. Such apparatus is well known and various forms thereof are shown in the following patents:

Allen 1,781,153 Allen 1,824,745 Eyer 2,043,241 Howe 2,076,944 Crist 2,297,346

For the practice of my method, such form of the capacitance measuring apparatus may be :selected as may be deemed most convenient and i accurate.

When an electrical resistance type wire strain gauge is cemented to the structure to be tested, the bond between the gauge and the structure must be perfect if the results are to be trusted.

-In the case of a metal structure, the bond is therefore checked according to the present invention as follows: The strain gauge and the metal structure are each separately connected to the `capacitance vbridge so thaththey will form the plates of a condenser. Theadhesive bond is considered to'be the dielectric.

Y Referring now to the drawings,

Fig. lis a diagrammatic showing of the arrangement of the condenser plates on the structure to be tested.

Y Fig. 2 is a plan View of the condenser plate 'arrangement shownk in Fig. I.

, Fig. 3 is a cross section similar to Fig. 2, showing the arrangement of condenser plates for testing diagonal or scarfed joint. Fig. 4 is a plan view showing a side-by-side arrangement of a condenser plate and a strain gauge on a xed structural member.

Fig. 5 is a cross section of Fig. 4 taken on the line 5-5 thereof.

In the figures, Ill is the structure the strength of which is to be ultimately tested; I I is a capacitance bridge, I3 is an electrical resistance type strain gauge,v 2B is the bond between the strain gauge and the structure I0, i5 and I5a are the strain gauge leads and (in Fig. 3) I3 is a lower condenser plate and Il an upper condenser plate. I8 and I9 are leads connecting condenser plates I6 and I'I respectively of their equivalent to the capacitance bridge II.

In Fig. 1 for example, Il) may be a metal plate 7 in the floor of a railroad bridge. The strain gauge I3y is cemented by means of an adhesive layer 20 to the Vuppensurf-ace of the door I while both leads I and I5a' of.the..strain gauge are connected to a single terminal of the capacitance bridge I I. Lead I 9 may be attached to the upper surface of the bridge floor at a small distance from the edge of the strain gauge I3. The approximate disposition of the wire 2| inside the strain gauge I3 is shown in Fig. 2, but this is not critical and 1 may be varied. The composite structure therecapacity is noted. Aninsulated prod 22 is now vpressed on the -top surfaceofstrain gauge I3and the capacity reading is noted while the pressure l is applied. No more pressure is used on the prod than is required to fully compress the coil spring 23 in its handle. It will require a known weight to compress the spring. TheY changev in reading of the bond capacitance is then noted. This change is compared with the change in capacitance produced in condensers formed by similar .strain gauges, similar cement and metal plates to which the gauges were bonded which were determined to be either properly or improperly bonded by mechanically loading the plates to which the strain gauges were cemented and noting the strain readings of the strain gauges and comparing these strain readings to strain readings obtained by other means. As an example, the following results are given to show values ci! variation in electrical capacitance indicating satisfactory and unsatisfactory bonds using the gauges and cements above mentioned.

Diderence in Capacity B ond Specimen Micromicro- Good. D

The exact point of diiierence between good and bad is selected, in view of the results sought and then applied as before and the instrument reading produced by the pressure is noted. This is due to the change in capacitance of the bond and is read directly without calculations.

Fig. 3 shows the method being applied to plywood, which is designated 20. In this method an upper condenser plate I'I and a lower condenser plate IB are rst applied to the wood 20 with only moderate pressure and the reading noted. Heavier predetermined pressure is then applied, preferably with an insulated or non-conducting pressure applicator, say a wood vise. The reading under such pressure is noted, and the difference determined. A high diierence is the sign of a weak bond.

The strength of a scarfed joint 24 can also be obtained with this method. The scarfed joint in ordinary lumber (not plywood) can most satis factorily be so tested, but the scarfed joint of plywood can also be tested if the correction to be applied for the change in capacity of the bond between the plies is rst ascertained. The calculations involved are simple and will be obvious to experts.

A side-by-side arrangement of condenser plates is the equivalent of the sandwich arrangement shown and described for Figs. 3 and 6. This arrangement is used for testing the bond to conducting materials. The distance between the nearest plate edge influences the condenser capacity. Such an arrangement is shown in Figs. 4 and 5. In this arrangement both leads I5 and I5a of the strain gauge I3 are connected to one lead I8 from the capacitance indicating instrument I I. Readings are made just as in the vertical arrangement of condenser plates.

Fig. 6 shows another arrangement for testing the bonds to substantially non-conducting materials such as plastics, lumber, etc. The strain Vgauge I3 is adhesively mounted on the top of the material to be tested and a condenser plate I6 mounted directly below the strain gauge I3 without adhesive. One lead I8 connects the strain gauge and another lead I9 connects the lower condenser plate IB to the capacitance indicating In this o. Bad.

instrument II. The capacitance of the entire condenser is measured, and the procedure is identical to that which was described above in connection with Fig. 1.

In the case of Fig. 6, a bonded material of say. aluminum and wood may be tested. In such case the aluminum takes the place of the strain gauge I3 and the wood takes the place of the material I0. A lower condenser plate I6 is applied to the material I0 as described and shown.

I claim as my invention:

1. The method of non-destructively testing adhesive bonds which comprises the step of making the bond at least part of the dielectric of an electrical condenser, ascertaining the capacitance of said condenser first substantially Without pressure and then with pressure on the plates thereof and determining the difference between the capacitances. t

- 2.`The method according to claim 1 in which the bonds of plywood are tested by placing the plywood between condenser plates.

3. The method according to claim 1 in which a scarfed joint cemented with adhesive is placed between the condenser plates.

4. The method according to claim 1 inwhich plywood having a cemented scarred joint is placed between the condenser plates and a correction is made for the change in capacity of the bonds between the plies as distinguished from the change in capacity of the scarfed joint.

5. The method according to claim 1 in which the bond of a bonded materialvof a metal and a substantial non-conductor are to be tested, with thesteps of making the metal one plate of an electrical condenser and applying another plate to the substantial non-conductor to make-a complete condenser.

V sive bonds which comprise the steps of making a bond at least part of the dielectric of an electrical condenser, connecting the condenser so made into one arm of a capacitance bridge, ascertaining the capacitance of said condenser iirst substantially without pressure and then with pressure on the plates thereof and determining the dierence between the capacitance values so obtained.

BERNARD D. HABER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 65 Number Name Date 1,939,883 Edwards Dec. 19, 1933 1,984,166 Walter Dec. 11, 1934 2,123,812 Stevens et al July l2, 1938 2,373,846 Olken Apr. 17, 1945 

